The reason behind this purge is SEC (U.S. Securities and Exchange Commission) demand to stop these coins from being offered by Bittrex, because they are securities.
Source: http://stockemperor.com/currencies/sec-purges-cryptocurrencies/
If these coins are regarded as securities, what is stopping them from delisting every damn coin on these exchanges?
The definition for crypto tokens seems pretty vague so far, so how do BTS differ from other altcoins to be classified as a security i wonder...